The present invention relates to mounting and connection devices and techniques for use with microelectronic elements such as semiconductor chips.
Complex microelectronic devices such as modern semiconductor chips require numerous connections to other electronic components. For example, a complex processor chip may require hundreds of connections to external devices.
Typically, microelectronic components such as chips are mounted on substrates such as circuit panels having electrical contacts, and the contacts on the chip are electrically connected to the contacts of the substrate. The substrate may be a circuit panel with internal circuitry connected to the contacts. The substrate may be adapted to accommodate other components, including additional chips. Also, the substrate may have pins or other connectors adapted to connect the contacts or internal circuitry of the substrate to a larger assembly, thereby connecting the chip to the larger assembly.
Connections between microelectronic elements and substrates must meet several demanding and often conflicting requirements. They must provide reliable, low-impedance electrical interconnections. They must also withstand stresses caused by thermal effects during manufacturing processes such as soldering. Other thermal effects occur during operation of the device. As the system operates, it evolves heat and the components of the system, including the chip and the substrate expand. When operation ceases, the components cool and contract. When the assembly is heated and cooled during manufacture or in operation, the chip and the substrate expand and contract at different rates, so that portions of the chip and substrate move relative to one another. Also, the chip and the substrate can warp as they are heated and cooled, causing further movement of the chip relative to the substrate. These and other effects cause repeated strain on electrical elements connecting the chip and the substrate. The interconnection system should withstand repeated thermal cycling without breakage of the electrical connections. The interconnection system should provide a compact assembly, and should be suitable for use with components having closely-spaced contacts. Moreover, the interconnection should be economical.
Various solutions have been proposed to meet these needs. In particular, as disclosed in U.S. Pat. Nos. 5,148,265; 5,148,266; 5,455,390 and in International Publication WO 96/02068, flexible leads may be provided between the contacts on a chip or other microelectronic element and the contact pads of a substrate. According to preferred embodiments taught in these documents, a compliant layer, such as an elastomer or a gel may be provided between the chip and the substrate. Flexible leads connecting the chip and substrate may extend through the compliant layer. In these preferred arrangements, the chip is mechanically decoupled from the substrate, so that the chip and substrate can expand and move independently of one another without excessive stress on the electrical connections between the chip contacts and the contact pads of the substrate. Moreover, the assemblies disposed in these patents and publications meet the other requirements discussed above. In certain preferred embodiments according to these documents, the chip and the interconnections to the substrate can occupy an area of the substrate about the same size as the chip itself.
Nonetheless, still further improvement would be desirable. For example, it would be desirable to provide additional connection components and methods which provide effective mechanical decoupling and high resistance to thermally induced stresses, while also providing low cost and high reliability.
The present invention addresses the foregoing needs.
One aspect of the present invention provides microelectronic assemblies. Assemblies according to this aspect of the invention desirably include first and second microelectronic elements having contacts thereon, and further include a compliant dielectric material having cavities therein. Masses of a conductive material are disposed in the cavities so that the masses of the fusible conductive material are electrically interconnected between contacts on the first microelectronic element and contacts on the second microelectronic element. Thus, each mass forms part or all of a conductor extending between contacts on the two elements. The conductive material may be a liquid or may be a fusible material adapted to liquify at a relatively low temperature, typically below about 125xc2x0 C. Preferably, the conductive material in each mass is contiguous with the compliant material and is contained by the compliant material, so that the conductive material remains in place when liquid. The compliant layer keeps the liquid masses associated with different sets of contacts separate from one another, and electrically insulates the masses from one another. The elements may have confronting surfaces bearing the contacts, and the compliant material may be in the form of a compliant layer disposed between the confronting surfaces. In this case, the masses of conductive material are also disposed between the confronting surfaces of the elements. Most preferably, the liquid masses are contiguous with contacts of one or both of the microelectronic elements, so that the liquid masses are contiguous with the microelectronic elements and contained by the microelectronic elements in conjunction with the compliant layer.
When the masses of conductive material are liquid, essentially no forces will be transmitted between the elements through the electrical conductors. Stated another way, the electrical conductors have spring constants at or close to zero and do not resist movement of the contacts on the microelectronic elements relative to one another. Preferably, the compliant dielectric layer also allows confronting portions of the microelectronic element surfaces to move relative to one another. Thus, the dielectric layer desirably is formed from a material such as an elastomer, gel, foam or other material having relatively low resistance to deformation. Preferred assemblies according to this aspect of the invention thus allow portions of the contact-bearing surfaces on the microelectronic elements to move relative to one another and thus compensate for movement and distortion. As further discussed below, the compliant connection between the microelectronic elements also helps to compensate for tolerances encountered during manufacturing. The compliant, flexible connection between the microelectronic elements can be provided even where each conductor has substantial cross-sectional area. Thus, low resistance, low impedance conductors can be utilized without impairing the flexible connection.
The conductive material desirably is liquid at temperatures within the range of temperatures encountered during normal operation of the microelectronic elements. Where the conductive material is a fusible material, it may have a melting temperature within or below the range of operating temperatures of the microelectronic elements. The fusible material may be in its solid state or in its liquid state when the assembly is inactive. During operation, the fusible material is wholly or partially liquid, and mechanical stress on the electrical connections is relieved. Moreover, when the fusible material melts, cracks or other defects in the conductive masses are repaired. Alternatively, the conductive material may be a fusible material which melts at temperatures slightly above the range of temperatures encountered during normal operation. In this case, the assembly relieves mechanical stress in the electrical connections, and repairs defects in the connections, when the assembly is exposed to high temperatures during abnormal operating conditions or during processing operations.
The first and second elements may be rigid or flexible. For example, the first element may include one or more semiconductor chips and the second element may include a rigid substrate such as a rigid circuit panel or another semiconductor chip. Alternatively, one or both of the elements may include a flexible dielectric sheet overlying a surface of the compliant layer. Each dielectric sheet has an interior surface facing toward the compliant layer and an exterior surface facing outwardly, away from the compliant layer. Each dielectric sheet has contacts on the interior surface and may also have terminals disposed on the exterior surface of the dielectric sheet electrically connected to the contacts. The exposed terminals may be bonded to a substrate to thereby electrically connect the contact pads of the substrate to the terminals and thus connect the substrate to the contacts, the liquid conductors and the opposing microelectronic element.
Other aspects of the invention provide methods of making microelectronic assemblies. A method in accordance with this aspect of the invention may include the step of providing first and second elements with confronting, spaced apart interior surfaces defining a space therebetween and contacts on the interior surfaces, together with masses of a fusible electrically conductive material as discussed above, in the space between the confronting interior surfaces so that each mass electrically connects a contact on the first element to a contact on the second element. A method according to this aspect of the invention desirably further includes the step of introducing a flowable material around the masses and between the confronting surfaces of the elements and curing the flowable material to form a compliant dielectric layer disposed between the confronting surfaces and intimately surrounding each mass of fusible material. Preferably, the masses of fusible material are maintained in a substantially solid condition while the flowable liquid material is introduced. After curing, the compliant dielectric material holds the elements together.
One or both elements may include a flexible dielectric sheet as aforesaid having an exterior surface facing away from the first element and having terminals on the exterior surface and contacts on an interior surface facing toward the other element. The method may further include the step of forcing the terminals into substantially coplanar disposition while maintaining the masses of fusible conductive material in an at least partially molten condition. Preferably, this step is performed prior to completion of cure of the flowable material, either before or after introduction of the flowable material into the space. One or both elements may include one or more semiconductor chips. For example, the first element may include a unitary wafer incorporating plural chips, whereas the second element may include a flexible sheet as discussed above. Each chip may be aligned with a portion of the sheet and the contacts on each chip may be connected by the masses of flowable conductive material to the terminals in the aligned portion of the sheet. The method according to this aspect of the invention may include the further step of severing individual portions of the sheet and wafer to form individual units, each including one or more chips and the portion of the sheet aligned therewith. The step of providing the elements and the masses may include the step of providing the masses attached to contacts on one of the elements and then juxtaposing the elements with one another and at least partially melting the masses to thereby bond the masses to the contacts on the other element. For example, where one of the elements is a wafer, the masses may be provided on the wafer, and the wafer may be juxtaposed with the opposing element, such as with a flexible sheet.
According to a further aspect of the invention, a method of making a microelectronic assembly may utilize a metallic plate having a plurality of masses of fusible material disposed at predetermined locations thereon. The metallic plate, with the masses thereon, may be juxtaposed with a microelectronic element, so that the masses are aligned with contacts on the microelectronic element, and the masses may be bonded to the contacts. After bonding the masses of fusible material to the contacts, a flowable material is injected between the plate and the microelectronic element and cured to form a compliant dielectric layer intimately surrounding the masses of fusible material. After the compliant dielectric layer is formed, the metallic plate is subdivided, preferably by etching the plate, to form separate portions connected to separate ones of the fusible masses. Each portion of the metallic plate may form an individual terminal assembly, mechanically decoupled from the other terminal assemblies and from the microelectronic element but electrically connected to the microelectronic element through the fusible masses.
As further discussed below, the microelectronic element may include a wafer or another large array of semiconductor chips. The array can be subdivided to form individual units, each including a chip with the associated terminal assemblies and fusible masses, together with a portion of the compliant layer. The predictable, isotropic thermal expansion properties of the metallic plate help to provide precise alignment of the fusible masses with the contacts on the microelectronic element. The step of providing the plate with the fusible masses thereon desirably includes the step of forming a layer on a first side of the plate from a material, such as a polymer, that is not wettable by the fusible material. The layer is provided with apertures in the locations where the fusible material masses are to be placed. The first side of the plate is exposed to the fusible material in molten condition so that drops of said fusible material adhere to the plate at the apertures. For example, a second side of the plate opposite from said first side may be covered by a protective coating, and the plate may be dipped into a bath of the fusible material.
Yet another aspect of the present invention provides methods of operating microelectronic assemblies having first and second elements and having electrical interconnections between the elements including masses of a conductive material, the assembly also having a compliant layer surrounding the masses of conductive material. The method according to this aspect of the invention desirably includes the step of transmitting signals between the elements through the masses, and maintaining the masses in an at least partially liquid state during some portion of the operation. The compliant layer contains the liquid masses. The method may include the steps of melting the masses during operation of the assembly, as by heat generated by the assembly during operation, and freezing the masses. Typically, the method includes the step of repeating the melting and freezing steps repeatedly, each time assembly is operated. The flexible connection provides compensation for thermal expansion of the elements during these cycles of operation. Moreover, defects in the masses such as cracks caused by metal fatigue are eliminated when the masses are melted. Yet another aspect of the invention provides methods of processing assemblies as aforesaid including the step of exposing the assembly to an elevated temperature sufficient to melt the masses. Here again, the compliant material contains the liquefied conductive material so that the masses remain in position.
These and other objects, features and advantages of the present invention will be more readily apparent from the detailed description of the preferred embodiments set forth below, taken in conjunction with the accompanying drawings.